Question about travelling to SEA

Hey guys! Myself and two other friends are planning a 3-4 month trip to SEA, Australia, and Europe. We are flying into Bangkok on Jan. 27th and are planning on (potentially) visiting Thailand, Vietnam, Cambodia, Laos, Philippines, and Indonesia over the course of two months and then flying to Australia. I was just looking for some advice in terms of what we should do about the one-way ticket situation. Ive encountered problems before when travelling to Europe for only having a one way ticket and definitely want to avoid getting holed up at customs for several hours again. So I was thinking about booking a cheap refundable flight to Vietnam out of Bangkok so that when I arrived in Thailand we wouldnt encounter any issues, and then following this logic for every other country we went to.

Has anyone ever been on a similar type of open-ended trip to SEA, and what did you do? Any help is appreciated! Thanks!